4 OSTEOLOGY. 



centre, ossification occurs at comparatively later periods in one or more separate 

 points, forming secondary or tertiary centres ; and the portions of bones so formed, 

 which remain united to the main part for a time by intervening cartilage, are termed 

 epiphyses. In many instances entire consolidation of the bone by the osseous union 

 of the epiphyses does not take place till the full size has been attained, and this may 

 be as late as the twenty-third or even the twenty-fifth year of life. 



In their outward form the bones present much diversity, but have been reduced 

 by anatomists to the following classes : 1. Long or cylindrical, such as the chief 

 bones of the limbs. These consist of a body or shaft, cylindrical or prismatic in 

 shape, and two extremities which are usually thicker than the shaft, and have smooth 

 cartilaginous surfaces for articulation with neighbouring bones. The shaft is 

 generally hollow and filled with marrow, by which sufficient size and strength are 

 attained without undue increase of weight. 2. Tabular or fiat bones, like the 

 scapula, ilium, and the bones forming the roof and sides of the skull. 3. Short 

 bones, which are more or less cubical or oblong, as in the carpus and tarsus. 

 4. Irregular or mixed bones, mostly situated symmetrically across the median plane 

 of the body, and often of a complex figure, such as the vertebrae. 



In these differently shaped bones the osseous substance occurs in two forms, viz., the 

 compact and the spongy. There is, however, no essential difference in structure or propertie3 

 between these beyond that of thickness or thinness of the component material. 



The surfaces of bones present various eminences, depressions, and other marks, to designate 

 which the following terms are in common use. Any marked bony prominence is called a 

 process or apophysis ; a slender, sharp, or pointed eminence is named a spine, or spinovs 

 process ; a blunt one a tubercle ; a broad and rough one a tuberosity. The terms crest, line, 

 and ridge are usually applied to a prominent border, or to an elevation running some way 

 along the surface of a bone. A head (caput, capitulum, or capitellum) is a rounded process 

 usually supported on a narrower part named the necli (cervix). The term condyle, somewhat 

 variously applied by anatomists, is most frequently employed to denote an eminence bearing 

 a rounded articular surface. 



The cavities and depressions of bones are very variously named. An aperture or perfora- 

 tion, when short, is & foramen; when continued some way as a passage it is a canal or 

 meatus. A narrow slit is a fissure, an open excavation or hollow in one bone or in several 

 together is a fossa. This term is also sometimes applied to the socket of a joint, as in the 

 fflenoid or shallower, and the cotyloid or deeper form of joint-cavity. Sinus and antrum are 

 names applied to considerable cavities in the interior of certain bones. Besides these, various 

 other terms are employed which do not require explanation, such as notch (incisura), groove, 

 furrow (sulcus), &c. 



The number of bones in the skeleton varies at different periods of life, some 

 which are originally distinct becoming united together as the process of ossification 

 advances. The following is a statement of the number usually reckoned as distinct 

 in middle life : 



Single bones. Pairs. Total. 



The vertebral column . . . 26 - ... 26 



Axial , The skull ..... 6 8 22 



Skeleton. 1 The hyoid bone . . . .1 ... 1 



^ The ribs and sternum ... 1 12 25 



Appendicular f The upper limbs . . .... 32 64 



Skeleton. ( The lower limbs ....... 31 62 



34 83 200 



Besides the bones included in the above enumeration, there exist likewise the three 

 pairs of auditory ossicles, and various bones formed in tendons and called sesamoid, 

 the most constant of which are, besides the patella and pisiform bone, reckoned in 

 the table above as limb-bones, a pair in each thumb and great toe. 



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